A helper class is a small, named style in the theme’s custom stylesheet. You add that name in the module editor, on the item’s CSS Class field or on the module’s Custom Classes field. The class changes only what its rule describes. Every other setting on the module stays in effect.
Use module settings first. Consider a helper class when the module has no field for the treatment, or the field exists but cannot change at a phone size. A helper class is also useful when the same treatment should be reused on many modules without pasting a one-off color or size into each one. Do not add a helper class for spacing, alignment, background, or a button style the module already has.
A label above a heading. The heading repeater can set a size and a color. It cannot say “this label is 28px on a desktop and 20px on a phone, in a gray that is not a theme color.” Put a class on that heading item. The class sets the desktop size, the phone size, and the light-surface color. On a dark background, leave the heading color on Auto so the label stays light.
Padding that should change on a phone only. Module outer spacing is one padding for every screen. Sometimes the band should run to the left and right edges on a phone, while the content inside stays inset. There is no separate mobile value for just those two sides. Put a class on the module. The rule removes the outer left and right padding under the phone breakpoint and leaves the desktop padding alone.
To add one, write the class in the theme stylesheet, with a short comment that says what it does and which breakpoint it uses. Then type that exact class name into the CSS Class or Custom Classes field. Check the page on desktop and on a phone. If a module setting can do the same job, use the setting and do not add the class.